Classical fluorescence microscopy is limited in resolution by the wavelength of light, referred to as the “diffraction limit”, which restricts lateral resolution to about 200 nm and axial resolution to about 500 nm at typical excitation and emission wavelengths when a sample emits fluorescence that is detected by the microscope. Confocal microscopy is an optical imaging technique used to increase optical resolution beyond the diffraction limit by using point illumination and a spatial pinhole arrangement to eliminate out-of-focus emission light from specimens that are thicker than that of the focal plane, thereby delivering images with 1.41 times the resolution than the diffraction limit by a method that requires tightly closing the pinhole. Unfortunately, closing the pinhole diminishes the signal level of the emitted light from the sample to such an extent as to make this particular method of super-resolution impractical. In addition, a confocal microscope must perfectly align the excitation from the microscope's illumination beam with the pinhole/detector, since a misaligned pinhole results in a reduced and weak light signal being detected as well as resulting in reduced axial optical sectioning of the sample itself. As such, misalignment of the confocal microscope can cause a reduction in the light signal.
A method for resolution enhancement for confocal microscopy has been found that uses an array of detectors, such as pixels in a camera image, wherein each of the detectors in the array produces a separate confocal image. If the array of detectors is sufficiently small, each of the formed confocal images can be equivalent to similar confocal images formed by a confocal microscope with a tightly closed pinhole such that 1.41 times the resolution of the diffraction-limited microscope is achieved when the confocal images are properly aligned. In addition, deconvolution provides a further increase in image resolution. However, this detector array arrangement is limited since only a single excitation point is scanned throughout a two-dimensional plane of the sample, which limits the speed the sample can be scanned and subsequent detection of the fluorescence emissions of the sample.
Another type of microscopy, referred to as structured illumination microscopy (SIM), illuminates a sample with spatially modulated excitation intensity, which is translated and rotated in different positions relative to the sample, with a wide-field image being taken at each translation and rotation. Processing the raw images appropriately results in a final image having double the lateral resolution of conventional wide-field microscopy. Although such SIM systems generate images with 2× the spatial resolution of a conventional microscope, there is still a sacrifice in temporal resolution when producing the final image, as time is required to acquire each of the multiple raw images. SIM may also be used to reject out-of-focus blur, known as “optical sectioning”. However, such optical-sectioning is performed computationally, and is thus subject to shot (Poisson) noise. SIM is thus inappropriate for thick or highly stained samples, when background fluorescence may cause this shot noise contribution to overwhelm the in-focus signal.
As such, there is a need in the art for a structured illumination microscopy system that produces a multi-focal excitation pattern of the sample for each high resolution image without sacrificing scanning speed, and that is resistant to the shot noise that may corrupt SIM images.